Polycrystalline aluminum thin film and optical recording medium

ABSTRACT

A polycrystalline aluminum thin film is made of polycrystals of an alloy of aluminum. The polycrystalline aluminum thin film includes a first additive which is distributed with even concentration over an inside of each crystal grain and an interface of the crystal grain and a second additive which is distributed with higher concentration in the interface of the crystal grain than in the inside of the crystal grain.

TECHNICAL FIELD

The present invention relates to a polycrystalline aluminum thin filmand further relates to the application of the polycrystalline aluminumthin film to an optical recording medium, a magnetic disc, an opticalmemory, a mirror, an electrode, and the like.

BACKGROUND ART

An optical recording medium such as a Blu-ray-R disc contains areflective film inside irrespective of its recording and reproductionmethods. This reflective film is generally made of aluminum, gold,silver, alloys thereof, or silicon. Taking a case of, for example, anoptical disc such as a CD or a DVD, a thin film made of an alloy ofaluminum or gold is used. A pure gold thin film or a pure silicon thinfilm is used in a semi-transparent film of the DVD. (Refer to JapanesePatent No. 2880190, Japanese Patent No. 2898112, Japanese Patent No.3365762, and Japanese Patent No. 3655907)

In an optical recording device which uses a blue laser with a wavelengthof approximately 400 nm for recording or reproducing information, a goldor silicon thin film does not have sufficient reflectivity as areflective film of a recording medium thereof because an absorptioncoefficient for blue light is high. An alloy of silver has sufficientreflectivity, but brings increase in cost.

The optical recording medium such as the CD, the DVD, and the Blu-raydisc is becoming denser and having high capacity, and hence thesophistication of the reflective film of the optical recording medium isrequired.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

Aluminum and an alloy of aluminum are superior to the alloy of silver interms of cost and ease of handling. It was very difficult, on the otherhand, to sufficiently reduce the diameter of a crystal grain of thesealuminum family reflective films with respect to the laser spot size ofthe optical recording device by the blue laser on the reflective filmof, in particular, the Blu-ray disc. Thus, a disc using the aluminumfamily reflective film had high noise in recording and reproduction, andcould not obtain sufficient recording and reproduction characteristics.

As the application of aluminum, alloys of aluminum adding Cu, Ta, or Mghave been used and studied as an electrode. Using the alloys, however,necessarily increased wiring resistance, and the compatibility betweenreduction in resistance and the restraint of hillock and migration wasdifficult.

An object of the present invention is to provide a polycrystallinealuminum thin film made of aluminum or an alloy of aluminum which issuperior in terms of cost and ease of handling and a recording medium orthe like which uses the polycrystalline aluminum thin film and haspreferable recording and reproduction characteristics in an opticalrecording and reproduction device using a short-wavelength laser.

Resolving Means/Means for Resolving the Problems

A polycrystalline aluminum thin film according to the present inventionis made of polycrystals of an alloy of aluminum. The polycrystallinealuminum thin film contains a first additive which is distributed witheven concentration over the inside of each crystal grain and theinterface of the crystal grain and a second additive which isdistributed with higher concentration in the interface of the crystalgrain than in the inside of the crystal grain.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an image of Al—Pd—SnO₂ thin film by a transmission electronmicroscope in a cross-sectional scan.

FIG. 2 is an image of Al—Ti thin film by the transmission electronmicroscope in the cross-sectional scan.

FIG. 3 is a sectional view of an optical disc.

EXPLANATION OF REFERENCE NUMERALS

-   1 . . . optical disc-   11 . . . substrate-   12 . . . reflective film-   13 . . . second protective layer-   14 . . . recording film-   15 . . . first protective layer-   16 . . . resin cover layer

DETAILED DESCRIPTION OF THE INVENTION

The occurrence of hillock and migration which was assumed in thedevelopment of a polycrystalline aluminum thin film made of an alloy ofaluminum was considered. As a result, stress occurs inside thepolycrystalline aluminum thin film due to the movement of a carrier andthe hysteresis of heat. It is assumed that the stress works as drivingforce to cause the diffusion of Al atoms along a grain boundary, andhence the hillock or the like is formed.

The inventor found that introducing an oxide in forming an alloy made itpossible to restrain the occurrence of the hillock or the like in thediffusion of the Al atoms along the grain boundary. The alloying couldnot help increasing the resistance of a wiring film. By adding aplatinum group element to Al, however, it became possible to obtain apolycrystalline aluminum thin film as a reflective film and a materialfor an electrode which had the function of light reflection or heatdissipation and the possibility of hillock prevention.

In the thin film deposited by sputtering, approximately evensuper-saturated solid solution is generally formed. It is conceivablethat the super-saturated solid solution is formed in the thin film madeof aluminum with the platinum group element. In such super-saturatedsolid solution, the platinum group element discharged into the grainboundary in the process of depositing a film inhibits the diffusion ofAl in the grain boundary and evenly alleviates the compression stress ofthe film made of Al-platinum group element. After a series of thermalhysteresis such as a film deposition process and the like, thesuper-saturated solid platinum group element has been almost completelydeposited on the grain boundary as an intermetallic compound, so that itis possible to prevent increase in resistance to a certain extent.

Thus, the inventor proposes a polycrystalline aluminum thin film whichcomprises a plurality of crystal grains having aluminum as the mainingredient and an interface section with high concentration for coveringthe crystal grain having aluminum as the main ingredient. Such apolycrystalline aluminum thin film satisfies any of the followingcharacteristics and conditions described in (1) to (4).

(1) A polycrystalline thin film made of an alloy of aluminum comprises afirst additive (single element or compound thereof) which exists insidea crystal grain having aluminum as the main ingredient and isdistributed with even concentration in the thin film and a secondadditive (single element) which is distributed with higher concentrationin the interface of the crystal grain having aluminum as the mainingredient than in the inside of the crystal grain.

(2) At least one of Sn, Ti, Nb, and oxides thereof is used as the firstadditive which exists inside the crystal grain and is distributed witheven concentration in the thin film. In the case where the oxide isused, a conductive or semiconducting oxide is available in addition tothe oxides of Sn, Ti, and Nb.

(3) An element which has a higher melting point than aluminum and isresistant to oxidizing (an element with lower ionicity than aluminum,and more preferably an element stable against water, vapor, and acidwith low oxidizing power) is used as the second additive which isdistributed with high concentration in the interface of the crystalgrain.

(4) an aluminum oxide layer exists in the surface of the polycrystallinethin film. The foregoing additives do not exist or exist in minutequantities in the aluminum oxide layer, and the foregoing additives areunevenly distributed in the interface between the aluminum oxide layerand a layer with not-oxidized aluminum as the main ingredient.

Example

Two kinds of polycrystalline aluminum thin films made of Al—Pd—SnO₂ andAl—Ti were formed on silicon substrates by sputtering for comparison.Sputtering power was 300 W, and the thicknesses of the polycrystallinealuminum thin films measured by a fluorescent X-ray analyzer (ZXS-100Smanufactured by Rigaku Electric Industry Co., Ltd) were 93 nm and 78 nm,respectively. To analyze the distribution of elements inside thesepolycrystalline aluminum thin films, the polycrystalline aluminum thinfilms were cut by concurrently using ion milling (600 seriesmanufactured by Gatan, Inc), and cross sections were observed using afield emission type electron microscope with an energy dispersive X-rayanalyzer (JEM-2100F manufactured by JEOL Ltd.). The abundance ratios ofeach element in the cross sections of the thin films were measured by anenergy dispersive X-ray spectroscopy, EDX.

FIG. 1 shows an image of the Al—Pd—SnO₂ thin film by a scanningtransmission electron microscope (STEM).

Table 1 shows the abundance ratio (standardized by atomicity) among Al,Pd, and Sn in positions indicated with numbers in FIG. 1. Argon (Ar)serving as a sputtering gas, carbon (C) which is likely to be derivedfrom pollution in the process of manufacturing electron microscopesamples, and oxygen (O) derived from oxidation were detected in additionto Al, Pd, and Sn, but these were left out of the calculation of theabundance ratio.

TABLE 1 Abundance ratio (atomic ratio) Analysis point Al Pd Sn 1 100.00.0 0.0 2 95.3 3.0 1.7 3 99.5 0.3 0.2 4 98.5 1.5 0.0 5 94.4 3.9 1.7 697.2 1.5 1.2

FIG. 2 shows an image of the Al—Ti thin film by the scanningtransmission electron microscope (STEM).

Table 2 shows the abundance ratio (standardized by atomicity) between Aland Ti in positions indicated with numbers in FIG. 2. Argon (Ar) servingas the sputtering gas, carbon (C) which is likely to be derived frompollution in the process of manufacturing electron microscope samples,and oxygen (O) derived from oxidation were detected in addition to Aland Ti, but these were left out of the calculation of the abundanceratio.

TABLE 2 Abundance ratio (atomic ratio) Analysis point Al Ti 1 100.0 0.02 98.8 1.2 3 99.2 0.8 4 99.1 0.9 5 98.6 1.4 6 99.2 0.8

As is apparent from the foregoing analysis results, in thepolycrystalline aluminum thin film made of Al—Pd—SnO₂, the abundanceratio of Pd is higher in a crystal grain boundary (a position of number4 of FIG. 1) than inside a crystal grain (a position of number 3 of FIG.1), and the abundance ratio of SnO₂ is almost the same. In thepolycrystalline aluminum thin film made of Al—Ti, on the other hand, theabundance ratio of Ti does not have significant difference in comparingbetween the inside of a crystal grain (a position of number 3 of FIG. 2)and a crystal grain boundary (a position of number 4 of FIG. 2). It isalso apparent that additive elements such as Pd, Sn, and Ti gather inspaces between an aluminum oxide layer in the surface of the thin filmand a not-oxidized layer (interface layer, positions of numbers 2 and 5in both of FIGS. 1 and 2).

When an oxide of a metal except for aluminum and an element (secondadditive element) which has a higher melting point than aluminum and isresistant to oxidizing are introduced into sputtering atmospheretogether with aluminum or an alloy of aluminum and sputtering is carriedout, the oxide and the second additive elements are scattered withdischarge and taken into the film. Such an oxide taken into a crystalgrain of aluminum or the alloy of aluminum composing the polycrystallinealuminum thin film causes the distortion of a crystal lattice and henceinhibits the growth of the aluminum crystal grain. Such a secondadditive element is deposited on the crystal interface of the aluminumcrystal grain, and inhibits the growth of the aluminum crystal grain bypreventing a wall of the aluminum crystal grain from being extended.Thus, the oxide and the second additive element inhibit the growth ofthe aluminum crystal grain in the process of depositing the film. Asdescribed above, it is possible to keep the crystal grain very small.Typically, the average diameter of the crystal grain of thepolycrystalline aluminum thin film including the additive elements wassmaller than 47 nm. Making the crystal grain minute can expect theeffects of restraining hillock and migration.

As described above, this embodiment provides the aluminum thin filmincluding two kinds of additives, that is, the first additive (singleelement or compound, for example, Ti, SiO₂, or the like) which existsinside the crystal having aluminum as the main ingredient and isdistributed in the thin film with even concentration and the secondadditive (single element, for example, Pd, Pt, Au, Ag, Ru, Rh, or thelike) which is distributed with higher concentration in the interface ofthe crystal grain than inside the crystal having aluminum as the mainingredient.

As the second additive which is distributed with higher concentration inthe interface of the crystal grain, an element which has the highermelting point than aluminum and is resistant to oxidizing such as Pd,Pt, Au, Ag, Ru, Rh, or the like is selected.

An aluminum oxide layer exists in the surface of the thin film, and theadditives do not exist or exist in minute quantities in the aluminumoxide layer. The foregoing additives are unevenly distributed in theinterface between the aluminum oxide layer and a layer with not-oxidizedaluminum as the main ingredient.

Furthermore, when measuring reflectivity with respect to therelationship between the amount of the second additive and thereflectivity in the polycrystalline aluminum thin film, the addition ofthe second additive causes reduction in the reflectivity. It wasconfirmed that the amount of adding Pd to the polycrystalline aluminumthin film was 8 atoms % in the case of allowing a 10% reduction in thereflectivity, more preferably 6 atoms % in the case of allowing an 8%reduction in the reflectivity, furthermore preferably 3 atoms % in thecase of allowing a 6% reduction in the reflectivity, and most preferably0.6 atoms % at which noise reduction came to maximum. Thus, it ispreferable to contain Pd at 0.6 to 8 atoms % in the polycrystallinealuminum thin film on the whole.

In a like manner, the amount of adding Au is 7 atoms % in the case ofallowing a 10% reduction in the reflectivity, more preferably 5 atoms %in the case of allowing an 8% reduction in the reflectivity, andfurthermore preferably 3 atoms % in the case of allowing a 6% reductionin the reflectivity. Furthermore, 1.5 atoms % at which noise reductioncomes to maximum is the most preferable. Thus, it is preferable tocontain Au at 1.5 to 7 atoms % in the polycrystalline aluminum thin filmon the whole.

In a like manner, the amount of adding Pt is 5 atoms % in the case ofallowing a 10% reduction in the reflectivity. Furthermore, 0.4 atoms %at which noise reduction comes to maximum is the most preferable. Thus,it is preferable to contain Pt at 0.4 to 5 atoms % in thepolycrystalline aluminum thin film on the whole.

Practical Example 1

A write-once-read-many optical disc (hereinafter referred to as a WORMdisc) will be described as one example though the optical recordingmedium according to the present invention is not limited thereto.

A WORM disc provided with an Al—Pd—SnO₂ reflective film was formed. Ascomparative samples, WORM discs provided with a pure Al reflective film,an Al—Pd reflective film, and an Al—SnO₂ reflective film, respectively,were formed.

Referring to FIG. 3, each of the WORM discs has multilayer structure inwhich a reflective film 12, a second protective layer 13, a recordingfilm 14, and a first protective layer 15 are laminated in this order ona disc-shaped substrate 11 by sputtering and then a resin cover layer 16is put on. The substrate 11 made of a polycarbonate resin has the shapeof a disc with a thickness of 1.1 mm and a diameter of 12 cm, and has aspiral groove with a pitch of 0.320 μm. On this substrate 11, thereflective film 12 made of Al, Al—Pd, Al—SnO₂ or Al—Pd—SnO₂, the secondprotective layer 13 made of ZnS—SiO₂, the recording film 14 made ofBi—Ge—N, and the first protective layer 15 made of ZnS—SiO₂ werelaminated in this order by sputtering. Sputtering power was 700 W or2000 W depending on a sputtering device and the like. Table 3 shows thelayers of the disc, the materials of the layers, and the thicknesses ofthe layers. Furthermore, a polycarbonate sheet was pasted thereon usinga UV curable resin as an adhesive to make a light incident sidesubstrate (cover layer) 16 with a thickness of 0.1 mm. It should beappreciated that light for recording or reproducing information isapplied on the recording film 14 from the side of the resin cover layer16.

TABLE 3 Layer Material Thickness Reflective film Al 50 nm Al—Pd Al—SnO₂Al—Pd—SnO₂ Second protective film ZnS—SiO₂ 20 nm Recording film Bi—Ge—N12 nm First protective film ZnS—SiO₂ 25 nm

In such four kinds of discs, a random pattern with 1 to 7 modulationswas recorded on a guide groove surface being in a convex shape withrespect to the side of light incident at a linear velocity of 4.92 m/swith the use of an optical head with a wavelength of 405 nm and anumerical aperture of an objective lens of 0.85. Multi-pulse was used inthis recording, and the width of a window was 15.15 nsec. Table 4 showstotal noise, recording LD power, and jitter after recording measured inthe four kinds of discs.

TABLE 4 Material Jitter (composition Recording after Sputtering atomicratio) Total noise power recording power Al −42.0 dB 5 mW 7.5% 2000 W (100%) Al—Pd −45.1 dB 5 mW 6.9% 700 W (95.9:4.1) Al—SnO₂ −41.9 dB 6 mW7.9% 700 W (99.87:0.13) −42.6 dB 5 mW 6.5% 2000 W  Al—SnO₂—Pd −45.6 dB 5mW 6.9% 700 W (96.32:0.13:3.55)

As a way to restrain noise during recording and reproduction with apractically usable level and improve recording and reproductioncharacteristics, it is preferable that the average diameter of a crystalgrain of the reflective film 12 be smaller than a laser spot size(diameter) d. To further improve the recording and reproductioncharacteristics, it is preferable that the average diameter of thecrystal grain be smaller than a half of the laser spot size d. Morepreferably, the average diameter of the crystal grain is smaller thanone-fifth of the laser spot size d, and most preferably is smaller thanone-tenth of the laser spot size d. If such a reflective film is used ina recording medium for optical recording and reproduction by a shortwavelength laser such as a blue laser, it is possible to obtain a stableand preferable recording characteristic.

To be more specific, a laser spot size (diameter) d in an opticalrecording device with a laser is expressed by d=l/NA with the use of alaser wavelength l and a numerical aperture NA. Taking a case of, forexample, l=400 nm and NA=0.85, d is 470 nm. The average diameter of acrystal grain of the reflective film 12 made of pure aluminum or analloy of aluminum containing an oxide of metal except for aluminumaccording to this invention is smaller than 47 nm being one-tenth of avalue of d described above.

The present invention does not limit the type or the number of theprotective layers 13 and 15. The protective layers 13 and 15 may be madeof, for example, a metal compound such as a metal nitride, a metaloxide, a metal carbide, and a metal sulfide or a mixture thereof,including ZnS and SiO₂.

In a like manner, the material of the recording film 14 is appropriatelychangeable. When the material of the recording film 14 is, for example,a phase change material such as SbTe, GeSbTe, GeSbBiTe, GeBiTe, andInAgSbTe, such a recording disc becomes a rewritable recording disc.When the recording film 14 is made of a dye film such as an azo dye, acyanine dye, and a phthalocyanine dye, the recording disc becomes anorganic dye type recording disc. In other words, the present inventionis widely available in recording mediums using an optical reflectivefilm. The present invention is available in, for example, a card typeoptical recording or optical magnetic recording medium which does nothave a disc shape. The present invention is also applicable to aradiating layer in a thermally assisted magnetic recording medium.

Co-sputtering by which a sputtering target is divided into a pluralityof areas based on materials like an alloy of aluminum and an oxide ofmetal except for aluminum, or aluminum, a metal element (or metalcompound), and an oxide of metal except for aluminum may be used. Inother words, an oxide of metal except for aluminum is introduced intosputtering atmosphere together with aluminum or an alloy of aluminum.Thus, the reflective film 12 is made of pure aluminum or the alloy ofaluminum into which the oxide of metal except for aluminum is taken.Accordingly, it is possible to keep its crystal grain small withoutreducing reflectivity. When the oxide of metal except for aluminum isintroduced into the sputtering atmosphere together with aluminum or thealloy of aluminum and sputtering is carried out, the oxide is scatteredwith discharge and is taken into the film. Such an oxide inhibits thegrowth of the crystal grain of aluminum or an alloy of aluminumcomposing the reflective film 12 during a deposition process, and henceit is possible to keep the crystal grain very small. It is generallyconceivable that adding an oxide to a reflective film is undesiredbecause reflectivity is reduced. The reflective film according to thepresent invention, however, can have preferable reflectivity andrecording and reproduction characteristics even if the oxide is added.This is because that the average diameter of the crystal grain of thealloy of aluminum becomes minute greatly contributes as opposed toreduction in the reflectivity by adding the oxide.

Practical Example 2

A read-only disc will be described as one example though the opticalrecording medium according to the present invention is not limitedthereto.

The read-only disc provided with an Al—Pd—SnO₂ reflective film and aread-only disc provided with an Al—Ti reflective film as a comparativeexample were formed.

Since the read-only disc is not recordable, the read-only disc has thesame structure as the foregoing practical example 1 except that thesecond protective layer 13, the recording film 14 made of Bi—Ge—N, andthe first protective layer 15 made of ZnS—SiO₂ are not formed. Asubstrate 11 made of a polycarbonate resin has the shape of a disc witha thickness of 1.1 mm and a diameter of 12 cm, and has a spiral pit rowwith a pitch of 0.320 μm. Recorded information is held in the pit row,and a random pattern with 1 to 7 modulations is recorded so that aminimum pit length becomes 0.149 μm. At this time, this disc has astorage capacity of 25 Gbytes. On this substrate 11, the reflective film12 made of Al—Pd—SnO₂ or Al—Ti as the comparative example was formed bysputtering. Sputtering power was 300 W, and the thickness of thereflective film 12 was 15 nm. A polycarbonate sheet was pasted on thereflective film 12 using a UV curable resin as an adhesive to make alight incident side substrate (cover layer) 16 with a thickness of 0.1mm. On the foregoing two kinds of discs, jitter was measured with theuse of an optical head having a wavelength of 405 nm and a numericalaperture of an objective lens of 0.85. Linear velocity was 4.92 m/s andreproduction LD power was 0.35 mW. Table 5 shows measurement results.

TABLE 5 Composition Reproduction (atomic ratio) jitter Disc with Al:Ti =5.5% Al—Ti reflection film 99.2:0.8 Disc with Al:Pd:SnO₂ = 5.2%Al—Pd—SnO₂ reflection film 98.3:1.5:0.2

It was considered that the quality of a signal depended on the shape ofthe pit raw formed in the substrate made of the polycarbonate resin orthe like because the recorded information was held in the pits. However,since a low noise material is used in the reflective film, as thisembodiment, the SN ratio (signal to noise ratio) of a reproductionsignal is improved, and it becomes possible to obtain more preferablereproduction jitter. When the reflective film 12 is made thick to obtainhigh reflectivity, the reflective film 12 deforms the shape of pits inthe resin substrate. Therefore, it is expected that structure accordingto the present invention, in which the diameter of crystal grain issmall and even, can bring more superior performance.

Accordingly, the polycrystalline aluminum thin film according to thepresent invention can be used as a reflective film for use in theapplication of an optical recording medium, a magnetic disc, an opticalmemory, and the like. The polycrystalline aluminum thin film isavailable as various kinds of reflective mirrors, as a matter of course.

Furthermore, the polycrystalline aluminum thin film according to thepresent invention is available as an electrode. As an example, there isan organic electroluminescence (herein after simply called EL too)device. A plurality of organic EL devices is formed on a display panelsubstrate in a predetermined pattern. Each of the organic EL devices isprovided with an organic material layer including a luminescent layer.The luminescent layer is composed of at least one thin film made of anelectroluminescent organic compound material which emits light inresponse to an applied electric current.

The organic EL device comprises a transparent electrode, an organic ELmedium, and a metal electrode which are laminated in this order on atransparent substrate. The organic EL medium may be, for example, asingle layer of an organic electroluminescent layer, a medium with threelayers of an organic positive hole carrying layer, an organicelectroluminescent layer, and an organic electron carrying layer, amedium with two layers of an organic positive hole carrying layer and anorganic electroluminescent layer, or a medium with laminated layers inwhich an electron or positive hole injection layer is appropriatelyinserted between the layers described above. The polycrystallinealuminum thin film according to the present invention is effectivelyusable as such a metal electrode.

When an auxiliary metal line with low resistance is used as an auxiliaryelectrode for wiring the transparent electrodes (anode) in a displaypanel in which a plurality of organic EL devices is arranged into amatrix to make resistance low, the polycrystalline aluminum thin film iseffectively usable as a wiring material for protecting the metal line.

1. A polycrystalline aluminum thin film made of polycrystals of an alloyof aluminum, comprising: a first additive which is distributed with evenconcentration over an inside of each crystal grain and an interface ofthe crystal grain; and a second additive which is distributed withhigher concentration in the interface of the crystal grain than in theinside of the crystal grain, wherein said first additive is at least oneof Ti and oxides thereof, and wherein said second additive is at leastone selected from the group consisting of Pd and Ru.
 2. Thepolycrystalline aluminum thin film according to claim 1 furthercomprising an aluminum oxide layer covering the whole of said crystalgrain and said interface thereof.
 3. The polycrystalline aluminum thinfilm according to claim 1, wherein an average diameter of the crystalgrain of said polycrystal of said alloy of aluminum is 47 nm or less. 4.An optical recording medium comprising said polycrystalline aluminumthin film according to claim
 1. 5. A mirror comprising saidpolycrystalline aluminum thin film according to claim
 1. 6. An electrodecomprising said polycrystalline aluminum thin film according to claim 1.7. A sputtering target comprising said polycrystalline aluminum thinfilm according to claim 1.